The Great White’s Ways
Scientists have long thought that California’s great white sharks spent their lives patrolling the chilly waters off the central coast, venturing only as far as Southern California to breed and deliver their young.
Nothing, it seemed, could coax these relative homebodies to stray far from their coastal hunting grounds, where they love nothing more than making a quick meal of blubbery seals and sea lions.
Now new research has shattered the image of the hardened predator with the photogenic jaws.
Apparently they are world travelers enjoying a softer, more cosmopolitan lifestyle that includes spending the winter in Hawaii or other tropical waters.
A team of California researchers announced that discovery last week in the journal Nature after performing the tricky task of attaching electronic tags to a half-dozen sharks as they surfaced for a feeding frenzy.
These sophisticated Pop-up Satellite Archival Tags, which record light, sea temperature and pressure every two minutes for up to a year, were key to the new understanding of sharks. The same technology, researchers say, can be used for deciphering the movements of numerous sea creatures, resolving a major difficulty in ocean research.
“The open ocean is almost a complete unknown,” said Barbara Block, a marine physiologist at Stanford University. “It’s much harder to track one of these animals in the ocean than, say, track a tiger on land.”
The 6-inch tags stay on the animal until a preset date, when a tiny computer activates a battery to release an electric current. The electricity, combined with salt water, rapidly corrodes the thin metal that holds the tag to the fish, allowing it to break free and pop to the surface. Once there, the tag transmits data to a computer via satellite. The data describe an animal’s location from moment to moment for the months it was tagged with an accuracy of about 10 kilometers.
In the case of the sharks, one tag surfaced just south of Maui, revealing that the creature had been hanging out there for four months. Other tags popped up in the open ocean, halfway between the tip of Baja and Hawaii.
Not only did the data reveal migrations far from shore, the information from the tags showed researchers that the sharks can spend considerable time in the open ocean, far from any coast, and can dive to almost half a mile below the surface.
“The results surprised all of us,” Block said.
*
Tuna Tagged First
The tagging technology was pioneered in 1997 to track bluefin tuna, the ocean’s most valuable fish. A single fish, its red flesh revered in sushi bars, routinely fetches $30,000 or more at auction in Tokyo.
In the Atlantic Ocean, authorities have worried that bluefin are vanishing. International officials have imposed quotas on what U.S. and Canadian fishermen can pull from the American side of the Atlantic. Europeans and the Japanese, meanwhile, continue to harvest bluefin with far fewer restrictions from the eastern Atlantic.
Block and her colleagues, after tagging hundreds of these fish, created a political splash recently by showing that the fish swam freely from one side of the Atlantic to the other. Their findings undermined the notion that the adult tuna of the eastern and western Atlantic constitute two wholly separate populations. That, in turn, raised questions about the wisdom of different rules for catching tuna in different parts of the same ocean.
Now these researchers at the Tuna Research and Conservation Center, a joint project of Stanford and the Monterey Bay Aquarium, are expanding use of the technology to different types of fish to unravel mysteries of the deep.
Teaming up with biologists at the Farallon and Ano Nuevo islands, off the coast near San Francisco, they focused on one of the most enthralling but least understood creatures: the great white shark.
Yet tagging the world’s largest predatory fish, which can reach 21 feet in length and weigh 4,800 pounds, is a bit more dicey than labeling a tuna.
In the case of tuna, researchers found they could hoist the fish aboard ship, cover its eyes with a wet chamois and pump seawater into its mouth to calm it while the tag was affixed.
The great white shark, which inspired the movie “Jaws,” is known to be less cooperative.
“It would be logistically difficult to catch and release one,” said Peter Pyle, a Farallones’ biologist who has studied great white sharks for more than a decade.
Pyle and independent researcher Scot Anderson have spent years studying sharks near the islands--racing their small boat toward slicks of blubber oil and blood left over from an attack on an elephant seal or sea lion.
With cameras mounted on poles, they have identified many sharks, noticing that females seem to show up around the islands every other year. Sharks of both genders seem to depart in late December, their destination unknown.
The two eagerly invited Andre M. Boustany, one of Block’s Stanford doctoral students and lead author of the Nature article, to join them on their 17-foot Boston whaler and help implant electronic tags in the sharks circling below.
The researchers used an 8-foot pole mounted with a barbed titanium dart, which was connected by a metal leader to the cylindrical electronic tag.
“The gunwales are mid-calf, so there’s not a lot holding you in the boat,” Boustany said. The sharks are “14 to 18 feet long, pretty intimidating at first.”
The researchers set the first tags to release after only a few weeks to provide evidence that the technology worked without disturbing the sharks.
Tags on four other sharks were set to release after four or six months.
When the releases came, they revealed behavior never before recorded in a great white shark.
One large male, nicknamed Tipfin, took about 40 days to make the 2,280-mile trip to the Hawaiian island of Kahoolawe, just south of Maui, averaging 43 miles a day.
He remained there basking in the 79-degree water all winter and spring. Tipfin returned to the chilly 50 to 57 degree water around the Farallones this November, where he was implanted with a nine-month tag.
The three others, two females and a male, migrated to a warm-water area of the Pacific hundreds of miles west of the tip of Baja California. They remained for months, never venturing near any coastline.
“Going into this, we expected white sharks to be coastal animals, feeding on seals,” said Burney J. Le Boeuf, a biologist at UC Santa Cruz. The sharks also were thought to be shallow swimmers, living in the top 90 feet or so of water.
*
Fast Track to Hawaii
But the results not only showed the sharks spending as much as five months a year in the open ocean, they also revealed dives of up to 2,000 feet below the surface.
Moreover, during their migration the sharks tended to swim either near the surface or at depths of 900 to 1,500 feet. That prompted speculation that they might have found favorable currents, sort of a Hawaii express, to help them cover the distance more efficiently.
Some biologists hypothesize that great whites, one of only a few warmblooded shark species that carry their young until birth, head toward the tropics for breeding.
Others suggest they may be looking for different kinds of prey. Most acknowledge they don’t really know.
“All this leads us to the larger question: What are they doing over there?” said Le Boeuf, an animal behavior expert who is studying sharks with Scott Davis, a doctoral student.
“I guess they like to ease up a little bit, like we do.”
